Yolk-shell-type CaO-based sorbents for CO 2 capture: assessing the role of nanostructuring for the stabilization of the cyclic CO 2 uptake.

Improving the cyclic CO 2 uptake stability of CaO-based solid sorbents can provide a means to lower CO 2 capture costs. Here, we develop nanostructured yolk(CaO)-shell(ZrO 2 ) sorbents with a high cyclic CO 2 uptake stability which outperform benchmark CaO nanoparticles after 20 cycles (0.17 g CO 2 g Sorbent -1 ) by more than 250% (0.61 g CO 2 g Sorbent -1 ), even under harsh calcination conditions ( i.e. 80 vol% CO 2 at 900 °C). By comparing the yolk-shell sorbents to core-shell sorbents, i.e. structures with an intimate contact between the stabilizing phase and CaO, we are able to identify the main mechanisms behind the stabilization of the CO 2 uptake. While a yolk-shell architecture stabilizes the morphology of single CaO nanoparticles over repeated cycling and minimizes the contact between the yolk and shell materials, core-shell architectures lead to the formation of a thick CaZrO 3 -shell around CaO particles, which limits CO 2 transport to unreacted CaO. Hence, yolk-shell architectures effectively delay CaZrO 3 formation which in turn increases the theoretically possible CO 2 uptake since CaZrO 3 is CO 2 -capture-inert. In addition, we observe that yolk-shell architectures also improved the carbonation kinetics in both the kinetic- and diffusion-controlled regimes leading to a significantly higher cyclic CO 2 uptake for yolk-shell-type sorbents.